High strength steels are employed on commercial aircraft components such as landing gears, flap tracks and other high load items. Conventionally, special purpose coatings such as chrome deposits for wear or cadmium deposits for corrosion protection are deposited electrolytically from aqueous electroplating baths. Electroplating tends to liberate hydrogen from the cathode, which can lead to hydrogen embrittlement unless removed through controlled baking after plating.
High-strength low alloy steels are sensitive to delayed, brittle failures at certain low stresses. Such failures have been attributed to the presence of hydrogen in the steel microstructure. The hydrogen can be introduced into the microstructure by reaction with water or aqueous solution, or, by electrochemically discharging hydrogen at the surface of the steel. Since high-strength steels have corrosion-resistant coatings that are applied by electroplating techniques, hydrogen is discharged onto the steel surface along with the corrosion-resistant coating. Therefore, the quantity of hydrogen deposited at the coating steel interface must be carefully monitored and controlled.
In order to bake out hydrogen from these steels, a plate deposit of specific structure and morphology is required in order to allow the hydrogen to pass through the plating. For this purpose the aircraft industry and others have used low embrittlement processes such as BAC5709, “Hard Chromium Plating” and BAC5804, “Low Hydrogen Embrittlement Cadmium-Titanium Alloy Plating”.
Cadmium-titanium electroplating of high strength low alloy steels provides suitable resistance to hydrogen embrittlement. A Cd—Ti alloy is electroplated onto high-strength steels under carefully controlled conditions. The resulting plated product is then heat treated at elevated temperatures to achieve an acceptable low hydrogen embrittlement level. It is believed that the porosity of the electroplated cadmium-titanium alloy is the key to the removal of the hydrogen during a subsequent heat treatment operation. It must be noted that the cadmium-titanium plating bath is sensitive to contamination, so care must be taken to achieve acceptable embrittlement characteristics. And toxic components in the bath lead to health and environmental problems. For example, the cadmium-titanium alloy plating bath contains cadmium and cyanide, which create disposal problems unless expensive waste treatment equipment is employed.
Zinc/nickel alloys have been suggested for electroplating onto steels to render them corrosion-resistant. Such zinc/nickel baths are free of cadmium and cyanide free and contain relatively non-toxic components. Some have provided acceptable corrosion protection, but most have disadvantages such as difficulty in controlling the process, and unacceptably short bath life.
There continues to be a need for an improved coated product having corrosion resistance, low hydrogen embrittlement and which is non-toxic, especially as it relates to high strength low alloy steels for the aerospace industry.